Transparent conductors are used in high-performance displays, photovoltaic, touchscreens, organic light emitting diodes (OLED), smart windows and solar cells where high transparency and conductivity are required. The market for such transparent conductors may reach $5.6 billion by 2015.
Presently, ITO is the dominant transparent conductor, providing the best known combination of transparency (80%) and sheet resistance (10Ω/□). However, ITO has several crucial drawbacks. Namely:                ITO is increasingly expensive due the scarcity of indium;        ITO has limited environmental chemical stability and finite permeability which can lead to device degradation;        ITO easily wears out or cracks when bending/compression is involved        ITO is not flexible and such cannot be used in flexible displays, solar cells and touch panels        
The possible replacements of ITO include metal grids, metallic nanowires, metal oxides and nanotubes, while none of them provides performance as good as ITO.
Graphene is novel type of two-dimensional material arranged in a hexagonal honeycomb structure. As an atomic layer membrane, graphene is highly transparent (97.3%) over wide wavelengths ranging from visible to near infrared (IR). Owing to its covalent carbon-carbon bonding, graphene is also one of the stiffest materials with a remarkably high Young's modulus of ˜1 TPa, yet stretchable and bendable at the same time, with a maximum stretchability of up 20%. The combination of its high transparency, wide-band optical tunability and excellent mechanical properties make graphene a very promising candidate for flexible electronics, optoelectronics and phonotics. The technical breakthrough of large-scale graphene synthesis has further accelerated the employment of graphene films as transparent electrodes.
To utilize graphene as transparent electrodes in optoelectronic devices such as solar cells, organic light emitting diodes, touch panels and displays, the key challenge is to reduce the sheet resistance to values comparable with indium tin oxide (ITO), which provides the best known combination of transparency (90%) and sheet resistance (<100Ω/□). To achieve ultralow sheet resistance, the typical prior art approach is by heavily doping graphene. This is because sheet resistance follows the Drude model as shown in the following:
  Rs  =            1      σ        =          1              ne        ⁢                                  ⁢        μ            where n is charge carrier concentration, e is fundamental element charge of electrons and μ is charge carrier mobility in graphene. Charge mobility of graphene is roughly a constant, which depends on sample fabrication procedures. Thus, effectively increasing carrier density n will directly decrease the sheet resistance value of graphene.
Currently, chemical doping has been shown to effectively reduce the sheet resistance of graphene. Using nitric acid (HNO3) doping, the lowest sheet resistance ˜125Ω/□ with 97.4% transmittance in large-scale monolayer graphene has been achieved. However, the introduced chemical dopants are not stable over time and a protective coating or encapsulation steps are required. Furthermore, although the decreasing sheet resistance using chemical doping may be sufficient for touch panels, it may not work for many other applications such as solar cell, light emitting diodes and large-scale displays. Such other applications may need sub-10Ω/□ at transparencies larger than 90%.